Env variables doc modified (#6755)
Env variables doc modified (#6755)

* Env variables doc modified

* correction after review

* correction after review

* addressed some of the review comments

* addressed review comments

jamesliu/mxnet,gautamkmr/incubator-mxnet,reminisce/mxnet,stefanhenneking/mxnet,DR08/mxnet,mbaijal/incubator-mxnet,fullfanta/mxnet,coder-james/mxnet,formath/mxnet,sergeykolychev/mxnet,Prasad9/incubator-mxnet,leezu/mxnet,saurabh3949/mxnet,yuruofeifei/mxnet,sxjscience/mxnet,Mega-DatA-Lab/mxnet,CodingCat/mxnet,CodingCat/mxnet,stefanhenneking/mxnet,ShownX/incubator-mxnet,smolix/incubator-mxnet,madjam/mxnet,hesseltuinhof/mxnet,TuSimple/mxnet,jennyzhang0215/incubator-mxnet,jiajiechen/mxnet,nicklhy/mxnet,leezu/mxnet,dmlc/mxnet,fullfanta/mxnet,thirdwing/mxnet,DickJC123/mxnet,Prasad9/incubator-mxnet,Ldpe2G/mxnet,ykim362/mxnet,sxjscience/mxnet,Northrend/mxnet,reminisce/mxnet,yajiedesign/mxnet,formath/mxnet,ForkedReposBak/mxnet,ptrendx/mxnet,dmlc/mxnet,indhub/mxnet,jennyzhang0215/incubator-mxnet,kkk669/mxnet,arank/mxnet,ptrendx/mxnet,madjam/mxnet,yuruofeifei/mxnet,ShownX/incubator-mxnet,madjam/mxnet,tornadomeet/mxnet,TuSimple/mxnet,nicklhy/mxnet,DR08/mxnet,kkk669/mxnet,stefanhenneking/mxnet,coder-james/mxnet,weleen/mxnet,ZihengJiang/mxnet,tornadomeet/mxnet,coder-james/mxnet,jennyzhang0215/incubator-mxnet,yajiedesign/mxnet,Mega-DatA-Lab/mxnet,gautamkmr/incubator-mxnet,jermainewang/mxnet,jennyzhang0215/incubator-mxnet,piiswrong/mxnet,saurabh3949/mxnet,TuSimple/mxnet,apaleyes/mxnet,precedenceguo/mxnet,rahul003/mxnet,Northrend/mxnet,thirdwing/mxnet,DR08/mxnet,TuSimple/mxnet,nicklhy/mxnet,hpi-xnor/BMXNet,arank/mxnet,Northrend/mxnet,indhub/mxnet,indhub/mxnet,crazy-cat/incubator-mxnet,xcgoner/dist-mxnet,hesseltuinhof/mxnet,CodingCat/mxnet,vikingMei/mxnet,navrasio/mxnet,jennyzhang0215/incubator-mxnet,precedenceguo/mxnet,yajiedesign/mxnet,solin319/incubator-mxnet,ptrendx/mxnet,dmlc/mxnet,mbaijal/incubator-mxnet,weleen/mxnet,tornadomeet/mxnet,navrasio/mxnet,Mega-DatA-Lab/mxnet,ptrendx/mxnet,ZihengJiang/mxnet,smolix/incubator-mxnet,solin319/incubator-mxnet,hesseltuinhof/mxnet,hotpxl/mxnet,wangyum/mxnet,larroy/mxnet,wangyum/mxnet,mbaijal/incubator-mxnet,DR08/mxnet,mbaijal/incubator-mxnet,crazy-cat/incubator-mxnet,lxn2/mxnet,TuSimple/mxnet,szha/mxnet,jermainewang/mxnet,precedenceguo/mxnet,vikingMei/mxnet,fullfanta/mxnet,precedenceguo/mxnet,weleen/mxnet,arank/mxnet,jennyzhang0215/incubator-mxnet,navrasio/mxnet,Northrend/mxnet,tornadomeet/mxnet,zhreshold/mxnet,yajiedesign/mxnet,larroy/mxnet,stefanhenneking/mxnet,navrasio/mxnet,tlby/mxnet,apache/incubator-mxnet,LinkHS/incubator-mxnet,nicklhy/mxnet,lxn2/mxnet,navrasio/mxnet,piiswrong/mxnet,leezu/mxnet,coder-james/mxnet,mbaijal/incubator-mxnet,kevinthesun/mxnet,reminisce/mxnet,navrasio/mxnet,Mega-DatA-Lab/mxnet,vikingMei/mxnet,jermainewang/mxnet,hotpxl/mxnet,CodingCat/mxnet,smolix/incubator-mxnet,kkk669/mxnet,dmlc/mxnet,EvanzzzZ/mxnet,jiajiechen/mxnet,smolix/incubator-mxnet,ForkedReposBak/mxnet,antoan2/incubator-mxnet,hotpxl/mxnet,vikingMei/mxnet,indhub/mxnet,hpi-xnor/BMXNet,EvanzzzZ/mxnet,Northrend/mxnet,thirdwing/mxnet,madjam/mxnet,zhreshold/mxnet,Ldpe2G/mxnet,thirdwing/mxnet,crazy-cat/incubator-mxnet,ykim362/mxnet,szha/mxnet,yuruofeifei/mxnet,arank/mxnet,reminisce/mxnet,rahul003/mxnet,ykim362/mxnet,jennyzhang0215/incubator-mxnet,LinkHS/incubator-mxnet,CodingCat/mxnet,hesseltuinhof/mxnet,antoan2/incubator-mxnet,ykim362/mxnet,ZihengJiang/mxnet,ShownX/incubator-mxnet,DR08/mxnet,ShownX/incubator-mxnet,yajiedesign/mxnet,precedenceguo/mxnet,eric-haibin-lin/mxnet,apaleyes/mxnet,ForkedReposBak/mxnet,coder-james/mxnet,hesseltuinhof/mxnet,wangyum/mxnet,luoyetx/mxnet,TuSimple/mxnet,yuruofeifei/mxnet,crazy-cat/incubator-mxnet,jamesliu/mxnet,solin319/incubator-mxnet,eric-haibin-lin/mxnet,ptrendx/mxnet,weleen/mxnet,larroy/mxnet,tlby/mxnet,stefanhenneking/mxnet,ShownX/incubator-mxnet,Mega-DatA-Lab/mxnet,DickJC123/mxnet,apaleyes/mxnet,formath/mxnet,sergeykolychev/mxnet,lxn2/mxnet,Mega-DatA-Lab/mxnet,hpi-xnor/BMXNet,hotpxl/mxnet,CodingCat/mxnet,tornadomeet/mxnet,apache/incubator-mxnet,indhub/mxnet,hesseltuinhof/mxnet,LinkHS/incubator-mxnet,jamesliu/mxnet,larroy/mxnet,smolix/incubator-mxnet,hotpxl/mxnet,smolix/incubator-mxnet,tlby/mxnet,rahul003/mxnet,kevinthesun/mxnet,Prasad9/incubator-mxnet,larroy/mxnet,solin319/incubator-mxnet,indhub/mxnet,ForkedReposBak/mxnet,wangyum/mxnet,antoan2/incubator-mxnet,smolix/incubator-mxnet,saurabh3949/mxnet,tornadomeet/mxnet,apache/incubator-mxnet,Ldpe2G/mxnet,ForkedReposBak/mxnet,dmlc/mxnet,szha/mxnet,LinkHS/incubator-mxnet,jermainewang/mxnet,DickJC123/mxnet,larroy/mxnet,ZihengJiang/mxnet,Prasad9/incubator-mxnet,fullfanta/mxnet,luoyetx/mxnet,DR08/mxnet,antoan2/incubator-mxnet,lxn2/mxnet,madjam/mxnet,solin319/incubator-mxnet,fullfanta/mxnet,ptrendx/mxnet,yuruofeifei/mxnet,formath/mxnet,reminisce/mxnet,kevinthesun/mxnet,eric-haibin-lin/mxnet,piiswrong/mxnet,Ldpe2G/mxnet,wangyum/mxnet,wangyum/mxnet,reminisce/mxnet,kkk669/mxnet,thirdwing/mxnet,nicklhy/mxnet,hesseltuinhof/mxnet,coder-james/mxnet,reminisce/mxnet,jermainewang/mxnet,luoyetx/mxnet,TuSimple/mxnet,sxjscience/mxnet,ZihengJiang/mxnet,lxn2/mxnet,hpi-xnor/BMXNet,arank/mxnet,solin319/incubator-mxnet,sergeykolychev/mxnet,zhreshold/mxnet,Northrend/mxnet,formath/mxnet,sxjscience/mxnet,stefanhenneking/mxnet,hesseltuinhof/mxnet,formath/mxnet,rahul003/mxnet,Ldpe2G/mxnet,Prasad9/incubator-mxnet,mbaijal/incubator-mxnet,gautamkmr/incubator-mxnet,arank/mxnet,CodingCat/mxnet,coder-james/mxnet,nicklhy/mxnet,sergeykolychev/mxnet,piiswrong/mxnet,rahul003/mxnet,crazy-cat/incubator-mxnet,precedenceguo/mxnet,rahul003/mxnet,EvanzzzZ/mxnet,hotpxl/mxnet,yuruofeifei/mxnet,zhreshold/mxnet,coder-james/mxnet,coder-james/mxnet,yuruofeifei/mxnet,jermainewang/mxnet,EvanzzzZ/mxnet,fullfanta/mxnet,ZihengJiang/mxnet,sergeykolychev/mxnet,vikingMei/mxnet,indhub/mxnet,zhreshold/mxnet,dmlc/mxnet,vikingMei/mxnet,Ldpe2G/mxnet,ykim362/mxnet,kkk669/mxnet,hotpxl/mxnet,zhreshold/mxnet,lxn2/mxnet,kkk669/mxnet,DR08/mxnet,mbaijal/incubator-mxnet,luoyetx/mxnet,szha/mxnet,ykim362/mxnet,xcgoner/dist-mxnet,dmlc/mxnet,arank/mxnet,stefanhenneking/mxnet,larroy/mxnet,EvanzzzZ/mxnet,ZihengJiang/mxnet,hpi-xnor/BMXNet,EvanzzzZ/mxnet,ykim362/mxnet,tlby/mxnet,tlby/mxnet,sergeykolychev/mxnet,DR08/mxnet,jiajiechen/mxnet,madjam/mxnet,arank/mxnet,jennyzhang0215/incubator-mxnet,Northrend/mxnet,Prasad9/incubator-mxnet,thirdwing/mxnet,kkk669/mxnet,kkk669/mxnet,gautamkmr/incubator-mxnet,kevinthesun/mxnet,tlby/mxnet,indhub/mxnet,Mega-DatA-Lab/mxnet,madjam/mxnet,leezu/mxnet,thirdwing/mxnet,szha/mxnet,luoyetx/mxnet,larroy/mxnet,jiajiechen/mxnet,eric-haibin-lin/mxnet,jiajiechen/mxnet,luoyetx/mxnet,indhub/mxnet,crazy-cat/incubator-mxnet,jiajiechen/mxnet,jermainewang/mxnet,jamesliu/mxnet,xcgoner/dist-mxnet,saurabh3949/mxnet,gautamkmr/incubator-mxnet,mbaijal/incubator-mxnet,eric-haibin-lin/mxnet,jamesliu/mxnet,xcgoner/dist-mxnet,hpi-xnor/BMXNet,smolix/incubator-mxnet,yajiedesign/mxnet,Prasad9/incubator-mxnet,szha/mxnet,yuruofeifei/mxnet,formath/mxnet,yuruofeifei/mxnet,gautamkmr/incubator-mxnet,fullfanta/mxnet,tlby/mxnet,ShownX/incubator-mxnet,EvanzzzZ/mxnet,lxn2/mxnet,lxn2/mxnet,piiswrong/mxnet,precedenceguo/mxnet,CodingCat/mxnet,xcgoner/dist-mxnet,precedenceguo/mxnet,weleen/mxnet,LinkHS/incubator-mxnet,thirdwing/mxnet,navrasio/mxnet,apaleyes/mxnet,ShownX/incubator-mxnet,zhreshold/mxnet,piiswrong/mxnet,szha/mxnet,tornadomeet/mxnet,solin319/incubator-mxnet,jiajiechen/mxnet,kevinthesun/mxnet,xcgoner/dist-mxnet,DickJC123/mxnet,hesseltuinhof/mxnet,piiswrong/mxnet,jamesliu/mxnet,nicklhy/mxnet,Prasad9/incubator-mxnet,precedenceguo/mxnet,antoan2/incubator-mxnet,LinkHS/incubator-mxnet,formath/mxnet,LinkHS/incubator-mxnet,Northrend/mxnet,ptrendx/mxnet,DR08/mxnet,wangyum/mxnet,jamesliu/mxnet,sergeykolychev/mxnet,ykim362/mxnet,fullfanta/mxnet,navrasio/mxnet,Mega-DatA-Lab/mxnet,madjam/mxnet,leezu/mxnet,eric-haibin-lin/mxnet,sxjscience/mxnet,kevinthesun/mxnet,zhreshold/mxnet,vikingMei/mxnet,nicklhy/mxnet,leezu/mxnet,apaleyes/mxnet,ZihengJiang/mxnet,hpi-xnor/BMXNet,szha/mxnet,solin319/incubator-mxnet,sxjscience/mxnet,gautamkmr/incubator-mxnet,yajiedesign/mxnet,stefanhenneking/mxnet,eric-haibin-lin/mxnet,ForkedReposBak/mxnet,eric-haibin-lin/mxnet,gautamkmr/incubator-mxnet,jiajiechen/mxnet,reminisce/mxnet,thirdwing/mxnet,Ldpe2G/mxnet,Prasad9/incubator-mxnet,lxn2/mxnet,reminisce/mxnet,saurabh3949/mxnet,TuSimple/mxnet,crazy-cat/incubator-mxnet,luoyetx/mxnet,jermainewang/mxnet,leezu/mxnet,Mega-DatA-Lab/mxnet,hpi-xnor/BMXNet,sergeykolychev/mxnet,madjam/mxnet,ykim362/mxnet,sxjscience/mxnet,LinkHS/incubator-mxnet,ShownX/incubator-mxnet,luoyetx/mxnet,wangyum/mxnet,apaleyes/mxnet,fullfanta/mxnet,LinkHS/incubator-mxnet,dmlc/mxnet,TuSimple/mxnet,antoan2/incubator-mxnet,stefanhenneking/mxnet,TuSimple/mxnet,hotpxl/mxnet,saurabh3949/mxnet,sergeykolychev/mxnet,tlby/mxnet,EvanzzzZ/mxnet,crazy-cat/incubator-mxnet,kkk669/mxnet,EvanzzzZ/mxnet,weleen/mxnet,saurabh3949/mxnet,saurabh3949/mxnet,crazy-cat/incubator-mxnet,smolix/incubator-mxnet,eric-haibin-lin/mxnet,xcgoner/dist-mxnet,jiajiechen/mxnet,leezu/mxnet,rahul003/mxnet,gautamkmr/incubator-mxnet,ShownX/incubator-mxnet,formath/mxnet,solin319/incubator-mxnet,apaleyes/mxnet,hpi-xnor/BMXNet,weleen/mxnet,kevinthesun/mxnet,nicklhy/mxnet,weleen/mxnet,jennyzhang0215/incubator-mxnet,larroy/mxnet,tlby/mxnet,rahul003/mxnet,yajiedesign/mxnet,Northrend/mxnet,jermainewang/mxnet,antoan2/incubator-mxnet,ptrendx/mxnet,apaleyes/mxnet,antoan2/incubator-mxnet,Ldpe2G/mxnet,yajiedesign/mxnet,arank/mxnet,reminisce/mxnet,ForkedReposBak/mxnet,ForkedReposBak/mxnet,zhreshold/mxnet,sxjscience/mxnet,dmlc/mxnet,kevinthesun/mxnet,piiswrong/mxnet,antoan2/incubator-mxnet,vikingMei/mxnet,luoyetx/mxnet,vikingMei/mxnet,weleen/mxnet,leezu/mxnet,szha/mxnet,tlby/mxnet,apache/incubator-mxnet,mbaijal/incubator-mxnet,tornadomeet/mxnet,wangyum/mxnet,xcgoner/dist-mxnet,xcgoner/dist-mxnet,jamesliu/mxnet,tornadomeet/mxnet,Ldpe2G/mxnet,ptrendx/mxnet,navrasio/mxnet,sxjscience/mxnet,jamesliu/mxnet,hotpxl/mxnet,piiswrong/mxnet,CodingCat/mxnet,rahul003/mxnet,kevinthesun/mxnet,ptrendx/mxnet,ForkedReposBak/mxnet,ZihengJiang/mxnet,saurabh3949/mxnet,apaleyes/mxnet